How do you use the Home-Run Bat in Super Smash Bros Brawl?

If you're talking about charging the Baseball bat, pick it up and like any other smash attack, press left/right and "A" ("2" with just the Wii-mote) at the same time. The character should go into a "wind up" animation and they and the bat should glow a bit. Try to hit the sandbag with the tip of the bat. Just a tip, no pun intended.